# How to Control the Speed of DC Shunt Motors?

#### DC Shunt Motor Speed Control Methods

In this post, we will discuss about various speed control methods of dc shunt motor.
The speed of the dc shunt motor is given by
$N \alpha \frac{Eb}{\phi } \$

but

Eb=V-IaRa

$N \alpha \frac{V - IaRa}{\phi } \$
Here,
Eb = Back EMF, in volts
Ia = Armature Current, in amps
Ra = Armature Resistance, in ohms
V = Terminal Voltage, in volts
Φ = useful flux per pole, in webers

Thus the speed of shunt motor can be changed by changing any one of the factors mentioned below:

1. Applied Voltage
2. Varying the resistance of armature circuit ( Armature control method)
3. Varying the flux per pole ( Field control method)

#### Armature Control Method:

• In this method a variable resistance is inserted in series with the armature coil as shown in the figure.
• As a result, the back emf is decreased and thus speed of motor is reduced.
• Since resistor R is in series with the armature, it carries the full armature current and results in a large power loss in large motors where a considerable speed reduction is required for long periods.
• This method can only provide speed below normal speed.
• In this method, speed can be varied upto rated speed. This is the disadvantage of this method.

#### Field Control Method:

• In this method, the external resistance R is connected in series with field winding as shown in figure. By this arrangement, the field current is reduced and the speed is increased.
• As shown in the above equation, the speed can be varied by varying the value of flux. This can be achieved by using a variable resistor in series with the field winding.
• This resistor is called as the shunt field regulator.
• When the value of resistance of the shunt field regulator is increased, the value of the field current(If) will be decreased.
• This results in a decrease in the value of flux, , and hence an increase in the speed, since n∝1/Φ
• In this method, the speed cannot be reduced below  rated speed.